<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box1{
            border:10px  red solid;
        }
        .box2{
            width: 100px;
            height: 200px;
            background-color: red;
            float:left;
        }
        .box3{
            clear: both;
            /* box3撑开box1 */
        }
        .box1::after{
            content: "";
            clear: both;
            display: block;
            /* 因为默认是行内元素所以需要转换一下 内容去掉可以直接无伤撑开 */
        }
    </style>
</head>
<body>
     <div class="box1">
         <div class="box2"></div>
         <div class="box3"></div>
     </div>
</body>
</html>